Reducing speckle noise in 3D mapping

Published on by Optics for Hire.

Random and pseudo-random pattern projectors used for 3D depth mapping typically use laser sources. This is because laser sources are coherent enough to diffract through a diffractive optical element (DOE). Non-coherent sources (LED, for example) can't be used for such purposes.

The use of highly coherent lasers does have a downside, related to the inherent laser speckle effect which reduces the accuracy and sharpness of the projected pattern. This effect can be critical for the high-resolution and/or high-density patterns.

OFH has developed methods to decrease speckle effect allowing for significant improvement of the projected pattern quality. The improvements allow for higher accuracy 3D-mapping systems without the use of software filtering. 

In the image below, taken with a long focal length camera shows the complex structure of the light spots made by a laser and DOE projection system.. The complexity of the Illuminance distribution means that depending on the location of a camera the intensity of a spot may appear differently. 

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An image of the same area received with help of the same camera but shifted on couple millimeters will differ from this image because of the speckle effect.

When the pattern is imaged with wide angle lenses, the image of the light spot (dot) covers just a couple of pixels. As result, images of spots are shifted on a couple of millimeters one relative another just can differ significantly.  The images below were created by stereo-system with 250mm base, using two 4mm lenses, two 5MP cameras, and a custom pattern projector. The differences in the spot intensity can be clearly seen.

The second pair of images shown below were taken with the same camera set up, but used OFH's speckle reduction methods. The highlight spots in the left and right image now have more uniform illuminance when compared to each other. 

The benefits of the reduced speckle can be seen in the 3D map below. This map was created with OFH software and shows a scene in our laboratory. The image on the left uses the initial laser pattern. The image on the right was created with the same software but with the additional speckle reduction method. The reduction in noise levels and higher accuracy may be seen.